In fabricating integrated circuits, photoresist is coated over semiconductor substrates and patterned through selective exposure to developing light and removal of either the developed or undeveloped portions. The patterned resist forms a mask used to extend the pattern into underlying layers, such as oxides or metal layers, by etching through the holes in the mask. Masks are also commonly used to selectively dope regions of the substrate by ion implantation. Once the mask has been employed, it is typically removed by an oxidation process. The oxidizing process is referred to in the industry as resist "stripping" or "ashing."
Increased throughput is a primary objective in the semiconductor industry, particularly in the current era of single-wafer processing. Any reduction in the time required for processing each substrate serially in single-wafer processing systems can lead to significant cost savings in a highly competitive industry. In the case of resist stripping, the rate of processing can be increased by supplying reactive oxygen free radicals to the substrate. For example, dissociation of oxygen-containing gases, such as diatomic oxygen gas (O.sub.2), results in atomic oxygen (O), also known as oxygen free radicals.
The addition of fluorine, in the form of NF.sub.3, CF.sub.4, SF.sub.6 or fluorine free radicals (F), often aids the stripping process where resist chemistry has been complicated by prior processing. For example, it is difficult to remove photoresist that has been subjected to ion implantation, such as that employed in electrically doping the semiconductor substrate through the mask. Similarly, reactive ion etch (RIE) through resist masks, particularly where metal is exposed during the etch, tends to form polymeric residue, which is also difficult to remove by oxidation alone. In each of these situations, application of fluorinated chemistries aids cleaning the resist and residue from the substrate. Fluorine is also commonly used in other cleaning or etching steps.
Maximizing the generation of oxygen (and/or fluorine) free radicals positively influences the rate at which resist can be stripped, thus increasing substrate throughput. Such free radicals are commonly produced by coupling energy from a microwave power source to oxygen-containing gas. Remote microwave plasma generators guide microwave energy produced in a magnetron through a waveguide to a resonant cavity or "applicator," where the energy is coupled to a gas flowing through the cavity. The gas is excited, thereby forming oxygen free radicals (O). Fluorine free radicals (F) are similarly formed when fluorine source gas is added to the flow. Common source gases include O.sub.2 for providing O, and NF.sub.3, CF.sub.4, SF.sub.6 or C.sub.2 F.sub.6 for providing F. Nitrogen (N.sub.2) forming gas (N.sub.2 /H.sub.2) is often added to the flow to increase particle kinetics and thereby improve the efficiency of radical generation.
While microwave radical generators can lead to significant improvements in ash rates, conventional technology remains somewhat limiting. The plasma ignited by the microwave power, for example, includes highly energetic ions, electrons and free radicals (e.g., O, F, N). While O and F free radicals are desirable for stripping and cleaning resist from the substrate, direct contact with other constituents of the plasma can damage the substrate and the process chamber. Additionally, the plasma emits ultraviolet (UV) radiation, which is also harmful to structures on the substrate.
Direct contact between the plasma and the process chamber can be avoided by providing a transport tube between the microwave cavity or applicator and the process chamber. The length of the tube is selected to encourage recombination of the more energetic particles along the length of the tube, forming stable, less damaging atoms and compounds. Less reactive F and O radicals reach the process chamber downstream of the microwave plasma source in greater proportions than the ions. Because the process chamber is located downstream of the plasma source, this arrangement is known as a chemical downstream etch (CDE) reactor. By creating a bend in the tube, the process chamber is kept out of direct line of sight with the plasma, such that harmful UV radiation from the glow discharge does not reach the substrate.
The tube itself, however, places several limitations on the CDE reactor. Conventionally, both the applicator and the transport tube are formed of quartz. Quartz exhibits advantageously low rates of O and F recombination, permitting these desired radicals to reach the process chamber while ions generated in the plasma source recombine. Unfortunately, quartz is highly susceptible to fluorine attack. Thus, the quartz transport tube and particularly the quartz applicator, which is subject to direct contact with the plasma, deteriorates rapidly and must be frequently replaced. Each replacement of the quartz tubing not only incurs the cost of the tubing itself, but more importantly leads to reactor downtime during tube replacement, and consequent reduction in substrate throughput.
An alternative material for applicators and/or transport tubes is sapphire (Al.sub.2 O.sub.3). While highly resistant to fluorine attack, sapphire tubes have their own shortcomings. For example, sapphire transport tubes exhibit much higher rates of O and F recombination as compared to quartz, resulting in lower ash rates. Additionally, sapphire is susceptible to cracking due to thermal stresses created by the energetic plasma, limiting the power which can be safely employed. Lower plasma power means less generation of free radicals, which in turn also reduces the ash rate. While employing single-crystal sapphire somewhat improves the strength of the tube relative to polycrystalline sapphire, safe power levels for single-crystal sapphire are still low compared to those which can be employed for quartz tubes. Moreover, bonding material at the joint between sapphire sections that create the bend in the transport tube, which prevents UV radiation from reaching the process chamber, is typically as susceptible to fluorine ion attack as is quartz.
Other limitations on the production of radicals in a conventional microwave plasma generator relate to the efficiency of the energy coupling mechanism. Much of the microwave power supplied by the magnetron is lost in power reflected back up the waveguide, where it is absorbed by an isolator module designed to protect the magnetron.
Energy also escapes where the applicator carries source gas in and free radicals out of the resonant cavity. The plasma-filled tube acts as a conductor along which microwave energy travels out of the cavity, thus effectively extending the plasma and reducing plasma density. In addition to reducing plasma density, and therefore reducing generation of radicals, the extension of the plasma also increases the risk of ions surviving to reach the process chamber and substrate housed therein. Microwave traps can confine such microwave leakage. For example, U.S. Pat. No. 5,498,308 to Kamarehi et al., entitled "Plasma Asher with Microwave Trap," discloses a resonant circuit trap. Even employing such traps, however, the plasma expands outside the plasma source cavity along the tube to the outer edges of the traps.
Accordingly, a need exists for more efficient microwave generators to improve resist ash rates.